Diffusion vacuum pump apparatus



May 11, 1965 w. G. BAcHLER 3,132,396

DIFFUSION VACUUM PUMP APPARATUS Filed Nov. 29, 1962 2 Sheets-Sheet 1,J-is 26 a 2 3 i it I6- X 1? INVENTOR. w. alien LER ATTORNEY May 11,1965 W. G. BACHLER DIFFUSION VACUUM PUMP APPARATUS Filed Nov. 29, 1962 2Sheets-Sheet 2 FIG 4 244c. 244c.

' TIME (om a Mom 0 DNM MA). MAx. 250c. 260 c. 260%.-

' -=(ouT)b ouT) b noun b on. TEMPERATURE NEAR BOT 0 o a A TOM WITH STIR-244 c MINUTE 244 c- RER'IN MOTION AND ARRESTED 4oN u ON)u (ON)u MAX, MAX250 c. 260 c. 260 c.

OUT)b (OUT)b Wounb 2T c. .L Z44PCTEMPLERATURE 25o WATTS 350WATTS45OWATTS F |G.5 60 (WATTSf 1000 FLUCTUATIONS OF TOTAL MINUTE PRESSUREFOR VARIOUS QUANTITIES OF PUMP FLUID AND HEATER INPUTS, NORMAL DIFFUSIONPUMP I200 I00 cm? on. zoom. on. A 3OOcm. O|L -H-IO' TORR (wATTs)- F I Gso'o FLUCTUATIONS OF TOTAL PRESSURE FOR VARIOUS QUANTITIES OF PUMP mooFLUID AND HEATER INPUTS,

MINUTE DIFFUSION PuMP WITH BUILT-IN STIRRER I200 INVENTOR, r W. BACHLER'BY N I400 9 -67 6 (No-8 TORR! lOOcm5 on. 2oom5o|L aoomi' olL ATTORNEYUnited States Patent Q 3,182,896 DIFFUSION VACUUM PUMP APPARATUS WernerG. .lliichler, Cologne, Germany, assignor to Leyhold Holding A.G., Zug,Switzerland Filed Nov. 29, 1962, Ser. No. 240,902 Claims. (Cl. 230-401)This invention relates to diflusion type vacuum pumps and moreparticularly relates to an improved boiler apparatus for suchpumps. a

The operation of high vacuum diffusion pumps is generally well known. Apumping fluid is evaporated in a heated boiler of the pump and theresulting vapor directed at supersonic velocity through a nozzle systemto be finally condensed on a cold surface. As the high speed vaporstream passes between the nozzle system and the condensing surface, itaccepts by diffusion gas molecules from the system being evacuated andcompresses these molecules into a higher pressure area which normallycommunicates with a mechanical backing pump. The liquid produced on thecondensing surface returns to the pump boiler to be reheated andre-evaporated.

The ultimate pressuresobtainable with such pumps is dependent upona-number of factors. Gas desorption from the walls of the vacuum system,degassing of demountable seals :and gasket material, virtual leaks andactual leaks, can all seriously limit the ultimate pressure obtainable,In addition, erratic and ineflicient operation of, the diffusion pumpsthemselves can seriously limit the base pressure obtainable within thevacuum system.

Tests have shown that much of the inconsistent and erratic performanceof diffusion pumps is a result of localized overheating in the pumpboiler which causes, during an accidental cavitation, the spontaneousdevelopment of vapor bubbles. These vapor bubbles then burst through thesurface of the pumping fluid producing eruptive boiling. The eruptiveboiling problem is less extensive when the pumping fluid contains largequantities of gas and dirt particles, since the individual moving gasmolecules and. dirt particles within the fluid tend to re lieve thepressure at local hot spots, thereby preventing the formation of largereruptive gas or vapor bubbles. However, the eruptive boiling problem canbecome quite substantial when the boiling fluid is relatively clean,i.e., freev of gas. This is precisely thecase after prolonged operationof a diffusion pump inthe ultra high vacuum range, which operation tendsto purify the pumping fluid by purging it of suspended gas molecules.

The result of localized eruptive boiling is to produce brief periodsduring which the thermal energy accumulated at the pumping fluid hotspots is released in addition to the energy continuously supplied by theheater element. This combined effect increases the pumping fluidevaporation rate causing the density of the vapor stream projecting fromthe nozzle to become abnormally high. The increased vapor density causesa corresponding increase in the backstreaming rate of pumping fluidmolecules into the high vacuum side of the vacuum system. This isespecially true of the light weight hydrocarbon molecules which areproduced at an increased rate during the brief. period of overheatingand eruptive boiling within the pump boiler.

Conversely, during non-eruptive boiling periods within the pumpboiler,the pumping fluid evaporation rate and density of the vapor streamleaving the nozzles is reduced. The lower vapor density allows gases onthe fore vacuum side with a high diffusion coefficient (for example,hydrogen and helium) to penetrate the vapor stream in the reversedirection and enter the high vacuum side of the 3,182,896 Pafented May11, 1955 ice vide a uniform vapor stream density which is high enough toprevent extensive back diffusion of high diffusion coeflicient gasmolecules into the high vacuum' side of the system and small enough toeliminate'excessive backstreaming of pumping fluid molecules.

. Many. heater designs have. been. utilized in an attempt to alleviateeruptive boilingproblems in diffusion pumps,.such as, for example,immersion of the heating element Within the pumping fluid, 1machining ofthe pump boilers internal surface to various configurationsdisadvantages, such as, for example, requirement of difficult vacuumseals, providing internal surfaces and compartments which are extremelydiflicult to clean, increasing the necessary dimensions of the pump,etc. i

An even more serious problem associated with the use of suchmechanicaldesigns is the fact that the magnitude of eruptive boilingwithin the pump boiler appears to depend upon a number of factors suchas geometry of the boiler, amount of power supplied by the pump heater,the specific load of the pump, the amount of pumping fluid, the degreeof the pumping fluid purity, etc. Because of the numerous variablefactors involved, it becomes extremely difficult to design a diffusionpump which will exhibit an absence of eruptive boiling over wideoperational ranges andfor extended periods of operational time. V :1

It is. therefore, the object of the present invention to provide 'adiffusion vacuum pump which exhibits an extremely uniform rate ofpumping fluid evaporation and a correspondingly uniform vapor streamdensity at the pump nozzle over a wide range of operating conditions.

One feature of the present inventionis the provision in a diffusionvacuum pump of a movable member for agitating the pumping fluid so as toestablish an even temperature distribution throughout the pumping fluid.

Another feature of this invention is the provision of a diffusion vacuumpump of the above featured type wherein the movable member is powered bya driving apparatus requiring no lead throughs in the vacuum walls ofthe pump. 7 e V Another feature of this invention is the provision of adiffusion vacuum pump of the above featured type wherein the drivingapparatus is aturbine device powered by the evaporated pumping fluidvapor.

Still another feature of this invention is the provision of a diffusionvacuum pump of the above featured type wherein the movable member is arota-ting vane positioned within the pumping fluid of the diffusion pumpboiler. Another feature of this invention is the provision in adiffusion vacuum pump of the above featured type of a support assemblyfor the pumping fluid agitator which is uniquely suited for use in adiffusion vacuum pump.

Another feature of this invention is the provision of a diffusion pumpof the above featured type wherein the turbine device includes aspecially designed statorand rotor assembly particularly suited for usein a diffusion pump wherein the pressure differential between the twosides of the turbine device is relatively small.

Another feature of this invention is the provision in a diffusion pumpof the above featured type of a braking device positioned within thepumping fluid and adapted to prevent the pumping fluid surface fromassuming a convex shape as a result of the centrifugal forces involved.V

These and other features and advantages of this invention will be moreclearly understood upon a perusal of the following specifications takenin conjunction with the accompanying drawings wherein:

FIG. 1 is a sectional elevation of a diffusion pump according to thisinvention;

FIG. 2 is a partial sectional plan view showing the rotating vane andbraking device positioned within the pumping fluid;

FIG. 3 is a partial sectional view showing in detail the driving turbinedevice;

FIG. 4 is a reproduction of a recording trace of the oil temperaturenear the bottom of the pump boiler with the agitator both in motion andat rest and for three different power inputs;

FIG. 5 is a reproduction of the recording traces representingfluctuations of total pressure at the high vacuum side of the pumpoperating without a pumping fluid agitator for three differentquantities of pump fluid and a variety of power inputs; and

FIG. 6 is a reproduction of the recording traces representingfluctuations of total pressure at the high vacuum side of the samediffusion pump operating under similar working conditions but utilizinga pumping fluid agitator according to the present invention.

Referring now to FIG. 1 there is shown a diffusion pump 11 having acylindrical wall 112 closed at its bottom end by a circular base plate13 adapted to be heated by a. conventional diffusion pump heater. Theopen upper end of cylindrical side wall 12 includes a flange 14 adaptedfor connection to a chamber it is desired to evacuate. A cooling jacket15 surrounds the upper portion of cylindrical side wall 12 while anaperture 16 in its lower portion communicates with a flanged exhausttubulation 17 adapted to communicate with a rough pumping system.

A plurality of annular chambers 18 are formed by concentric hollow tubes19 mounted on and hermetically sealed tothe heater base plate 13. Movinginwardly each successive concentric hollow tube 19 is longer than thepreceding outer one and each is capped by a vapor nozzle assembly 21.The lower portion of the diffusion pump 11 is filled with a pumpingfluid such as, for example, oil to a level indicated by the dotted line22, thereby serving as a boiler divided into a plurality offractionating compartments 23 by the hollow cylindrical tubes 18. Thecenter hollow tube 24 does not extend all the way to the bottom of thediffusion pump 11 so that the innermost fractionating compartment 25 isin communication with innermost annular hollow chamber 18 as well aswith the interior 26 of center hollow tube 24.

The operation of the diffusion pump 11 is generally well known whereinthe pumping fluid 22 is heated to evaporation and the resulting vaporrises to be directed through nozzle assembly 21. The escaping vapor isthen condensed on the cold surface of the diffusion pump wall 12 afterwhich it returns to the fractionating compartments 23 to be againreheated and evaporated. The use of fractionating compartments whereinthe more volatile constituents of the pumping fluid 22 are dischargedonly through the outer and higher pressure nozzles of the diffusion pump11 is also well known.

Referring now to FIGS. 1-3, the pumping fluid agitator device 30 of thisinvention is shown within the innermost fractionating compartment 25.Rotatively mounted within the innermost fractionating compartment 25 isthe shaft 31 which lies perpendicular to the circular base plate 13 andwhose pointed lower end rests at the center thereof. The shaft 31 issupported within central apertures of the mounting bar 32 and the stator33 which are in turn secured to the inner wall of innermost hollow tube19. Mounted on the lower portion of and for rotation with the shaft 31is the agitating vane 34 compris-' ing a plurality of radially extendingarms 35. Mounted i on the upper portion of the shaft 31 directly abovethe stator 33 is the rotor 36.

Each of the stator 33 and the rotor 36 is a circular plate having cutpie shaped sections therein bent at an angle to the original plane ofthe plate. The pie shaped blades 37 of the stator 33 are bent in adirection opposite to that of the rotor blades 38. Therefore risingpumping fluid vapor within the innermost fractionating compartment 25 isdirected by the stator blades 37 directly against the rotor blades 38 soas to cause rotation thereof. The rotation of rotor 36 produces rotationof the shaft 31 and the agitating vane 34 resulting in agitation of thepumping fluid 22.

The. smooth stirring of the pumping fluid by the agitating vane 34prevents the development of hot spots and the resulting erraticperformance discussed above. .Similar agitating mechanisms can also beused in the outer fractionating compartments 23. However, the embodimentshown is preferred because of its simplicity and also because theproblems associated with eruptive boiling are of much lesser importancein the fractionating compartments which are not associated with the highvacuum nozzles. It is also preferable that the agitating vane 34 belocated as near the base plate 13 as possible so as to agitate theextremely hot pumping fluid adjacent to the base plate surface. Also theblades of agitating vanes 34 should preferably be completely coveredwith the pumping fluid 22 so as to prevent splashing thereof.

An importantfeature of the agitating device 30 is the use of both astator 33 for directing the rising vapors and a driven rotor 36 forproviding motion. Tests have shown that the use of a rotor aloneproduces much less desirable results because the difference in gaspressure below and above the turbine are insufficient to providesatisfactory motive power. Also, the use of both a stator and a rotorprovides a unitary turbine device which is optically dense in thevertical direction except during-the brief periods of operation when theopenings between successive blades in each unit are aligned in thevertical direction. Since these periods comprise a small percentage oftotal operating time and since the openings themselves includea smallpercentage of the total surface area of each unit, the turbine devicewill function effectively as a barrier to drops of pumping fluid whichare carried along with the rising vapor. That is; a very largepercentage of such rising pumping fluid drops will be intercepted by theturbine device and prevented from reaching the vapor nozzle assembly 21.

The use of braking arms 39 is another extremely important feature ofthis invention. As shown in FIGS. 1-3 the plurality of flat rectangularbraking blades 39 are secured to the inner wall of the innermost hollowtable 19 immediately above the agitating vane 34. The braking blades arepreferably positioned so as to project partly into the pumping fluid andpartly above, as shown in FIG. 1. The braking blades 39 retard circularmovement of the pumping fluid 22 thereby reducing the appliedcentrifugal forces and preventing the surface of the stirred pumpingfluid 22 from taking an undesirable convex shape. The smooth pumpingfluid surface maintained by the braking plates greatly facilitates thesmooth performance of the diffusion pump 11.

The use of the shaft 31 with a pointed end rotating on the surface ofbase plate 34 and the simple apertured'supports 32 and 33 is alsopreferable over other more complicated designs such as,'for example;bearing supports. The support mechanism shown is extremely simple and ismuch less sensitive to failure or impaired operation as a result ofclogging by dirt and other contamination within the pump boiler.

Referring now to FIG. 4, the reproduced recording traces show theeffectiveness of the present invention in the distribution of heatthroughout the pumping fluid (oil was used as the pumping fluid medium).The curves show the measured temperature at the bottom of heated baseplate 13 for a variety of input powers. The agitating device was oh andon at one minute intervals with the points (a) indicating time agitatorwas started and points (b) representing times when motion of theagitator was discontinued. The curve for an input power of 250 wattsshows a maximum temperature of approximately 250 C. with the agitatingdevice not operating and a temperature of 240 C. with the agitatingdevice operating. The curve for 350 watts input power shows a Widelyfluctuating temperature with a maximum of about 260 C. for thenon-operating agitator and a relatively constant temperature of 244 C.with the agitator operating. The curve for 450 watts input power alsoshows erratic temperature variations with a maximum neat 260 C. for anoperating agitator and a relatively constant temperature of 244 C. withthe agitator operating.

Thus, the temperature at the bottom of the diffusion pump boiler wasmaintained much more constant and up to 16 C. in temperature cooler withthe agitating device of the present invention. There is also reason toassume that local temperature differences in the two cases were evenlarger.

The reproduced recording traces shown in FIGS. 5 and 6 indicate theeffectiveness of the present invention in eliminating the erraticpressure fluctuations commonly found in diffusion pumps. In the curves,time is shown on the vertical scale and pressure at the high vacuum sideof the pump on the horizontal scale with the vertical center of eachcurve representing 1 10 Torr. As shown in FIG. 5, a diffusion pumpWithout the agitating device of the present invention was operated withvarious amounts of pumping fluid oil and at input power ranging from 600to 1400 watts. The curve for the pump operating with 100 cubiccentimeters of pumping fluid shows extremely wide pressure fluctuationsin all except the 600 Watt input power range. The same pump operatingwith 200 cubic centimeters of pumping fluid oil shows wide pressurefluctuations over the entire range of input power. The curve for thesame diffusion pump operating with 300 cubic centimeters of pumpingfluid oil shows relatively constant pressure performance in the 600,1200 and 1400 input watt range but extremly wide pressure variations inthe 800 and 1000 watt ranges.

FIG. 6 shows similar reproduced curves for the same diffusion pumputilizing the agitating device of the pres ent invention. As shown inthe curves for each of the pumping fluid oil quantities, a relativelysmooth pressure performance was obtained over the entire range of inputpower.

Thus, the present invention provides a vacuum diffusion pump having anextremely uniform rate of pumping fluid evaporation and acorrespondingly uniform pressure performance. Furthermore, such aperformance is obtained with apparatus which is relatively inexpensiveto produce and is simple and trouble-free in use.

It will be undertsood that various changes in details, materials, stepsand arrangements of parts, which have been herein described andillustrated in order to explain the nature of the invention, may be madeby those skilled in the art within the principle and scope of theinvention as expressed in the appended claims.

What is claimed is:

1. A vacuum diffusion pump apparatus comprising a sealed outer wallhaving an opening adapted for connection to a chamber to be evacuatedand an exhaust aperture, a boiler section enclosed by the lower portionof said sealed outer wall and adapted to contain a pool of pumpingfluid, heater means for applying heat to said boiler section so as toheat and cause evaporation of the pumping fluid contained therein, avapor direction means supported by said outer wall and extending intosaid boiler section, said vapor direction means including a nozzleassembly adapted to direct evaporating pumping 6 fluid against the innersurface of said outer wall, and movable means so positioned within saidboiler section as to be substantially covered by the pumping fluid pooland to cause agitation thereof.

2. Apparatus according to claim 1 wherein said movable means comprises aplurality of rotatable arms adapted to be covered by the pumping fluid.

3. Apparatus according to claim 1 including mechanical braking meanspositioned within said boiler portion and adapted to retard circulatorymovement of the pumping fluid.

4. Apparatus according to claim 1 including a driving means positionedwithin said boiler section for imparting motion to said movable means,and a turbine device included in said driving means adapted to be drivenby the stream of vapor provided by the evaporating pumping fluid.

5. Apparatus according to claim 4 wherein said movable means comprises aplurality of rotatable arms adapted to be covered by the pumping fluidand to produce circular movement thereof.

6. Apparatus according to claim 5 including mechanical braking meanspositioned within said boiler portion and adapted to retard circularmovement of the pumping fluid.

7. Apparatus according to claim 4 wherein said turbuine device ispositioned between said nozzle assembly and the pumping fluid.

8. Apparatus according to claim 4 wherein said turbine device comprisesa rotor and a stator, said stator adapted to direct the stream of vaporagainst said rotor, and said rotor adapted to provide motion to saidmovable member.

9. Apparatus according to claim 8 wherein said rotor and said stator areadjacent each other so as to form a double barrier between said nozzleassembly and the pumping fluid thereby serving as an effective screen torising drops of pumping fluid.

10. Apparatus according to claim 9 including mechanical braking meanspositioned within said boiler portion and adapted to retard circulatorymovement of the pumping fluid.

11. Apparatus according to claim 9 wherein said movoable means comprisesa plurality of rotatable arms adapted to be covered by the pumpingfluid.

12. Apparatus according to claim 9 including a rotatable shaft joiningsaid rotor and said movable means, said rotatable shaft having a pointedend adapted to bear gainst the inner side of said sealed outer wall.

13. Apparatus according to claim 12 including mechanical braking meanspositioned within said boiler portion and adapted to retard circulatorymovement of the pumping fluid.

14. Apparatus according to claim 12 wherein said movable means comprisesa plurality of rotatable arms adapted to be covered by the pumping fluidand to'produce circular movement thereof.

15. Apparatus according to claim 14 including mechanical braking meanspositioned within said boiler portion and adapted to retard circularmovement of the pumping fluid.

References Cited by the Examiner UNITED STATES PATENTS 1,683,949 9/28Bergdoll 103-89 2,521,345 9/50 Cortright 230101 FOREIGN PATENTS1,202,482 7/59 France.

781,159 8/57 Great Britain.

LAURENCE V. EFNER, Primary Examiner.

WARREN E. COLEMAN, Examiner.

1. A VACUUM DIFFUSION PUMP APPARATUS COMPRISING A SEALED OUTER WALLHAVING AN OPENING ADAPTED FOR CONNECTION TO A CHAMBER TO BE EVACUATEDAND AN EXHAUST APERTURE, A BOILER SECTION ENCLOSED BY THE LOWER PORTIONOF SAID SEALED OUTER WALL AND ADAPTED TO CONTAIN A POOL OF PUMPINGFLUID, HEATER MEANS FOR APPLYING HEAT TO SAID BOILER SECTION SO AS TOHEAT AND CAUSE EVAPORATION OF THE PUMPING FLUID CONTAINED THEREIN, AVAPOR DIRECTION MEANS SUPPORTED BY SAID OUTER WALL AND EXTENDING INTOSAID BOILER SECTION, SAID VAPOR DIRECTION MEANS INCLUDING A NOZZLEASSEMBLY ADAPTED TO DIRECT EVAPORATING PUMPING FLUID AGAINST THE INNERSURFACE OF SAID OUTER WALL, AND MOVABLE MEANS SO POSITIONED WITHIN SAIDBOILER SECTION AS TO SUBSTANTIALLY COVERED BY THE PUMPING FLUID POOL ANDTO CAUSE AGITATION THEREOF.